Since the functions of the electronic products on the market are reinforced, the power supply is demanded to provide relatively greater output energy as a response. At the same time, the products are demanded to have smaller volume and lighter weight. These trends form two requirements towards the power converters: raising the power density and improving the conversion efficiency. Based on these two requirements, a large amount of active-clamp forward-flyback converters and their derivative circuits are applied to various power supply products due to their advantages of transmitting energy to the output continuously and having soft-switching function for the power switch.
FIG. 1 shows a circuit diagram of a conventional active-clamp forward-flyback converter, which has a DC power source Vin for providing a DC input voltage, a main switch S1, an auxiliary switch S2, a resonant inductor Lr, a resonant capacitor Cr (contributed by the sum of the parasitic capacitances of S1 and S2), a clamp capacitor Cc, a transformer Tr having a primary winding N1, a first secondary winding N2 and a second secondary winding N3 (the turn ratio of N1, N2 and N3 are: n1:n2:n3), a first diode D1, a second diode D2, an output inductor Lo, an output capacitor Co, and a load RL. The active-clamp forward-flyback converter can supply a stable DC voltage Vo. Moreover, the secondary side of transformer Tr with the central-tapped design could transfer energy to the output during the turn-on and the turn-off period of the main switch S1 such that the circuit has a larger energy processing capability. Meanwhile, current could be continuously outputted during a switching period. This feature produces smaller current ripples on the output capacitor such that the capacitance of output filter is smaller. Besides, the incorporation of the active-clamp circuit into the forward-flyback topology provides three main functions: clamping the voltage spike across switch S1 while this switch is turned off, magnetic reset of transformer, and achieving zero-voltage-switching (ZVS) for the power switches. Also, ZVS function could effectively decreases the switching losses of the power switches such that higher switching frequency could be chosen to reduce the volumes and weights of the passive elements.
The ZVS of main switch S1 of the active-clamp forward-flyback converter happens in predetermined dead time. This small interval is arranged between after auxiliary switch S2 is turned off and before main switch S1 is turned on. Within this interval, the current iS1 flowing through the main switch S1 resonates to the negative direction so as to discharge the parasitic capacitance of switches to be zero value, and then force the anti-parallel diode of main switch S1 to conduct. When the anti-parallel diode of main switch S1 is conducting, main switch S1 is turned on to achieve ZVS operation. However, in the actual circumstance, the ZVS of main switch S1 could not be smoothly achieved as aforementioned. The reason is explained as follows. FIG. 2 is a circuit diagram showing discharge process of the parasitic capacitance of switches of the active-clamp forward-flyback converter. At the moment, the current of the second secondary winding N3, iN3, is decreased gradually and the current of the first secondary winding N2, iN2, is increased gradually. With the increase of iN2, the current of the primary winding N1, iN1, will turn into flowing into the dot from flowing out the dot according to Ampere's law. When the current iN1 flows into the dot, the current iS1 rapidly turns into small and correspondingly the discharging speed of the parasitic capacitance of switches Cr is dramatically decreased. Due to the aforesaid feature, VCr could not be decreased to zero before main switch S1 is turned on. As a result, the ZVS could not be effectively achieved when main switch S1 is turned on, which is detrimental to conversion efficiency improvement and this drawback becomes serious under the heavy load condition. To overcome this drawback, the increasing speed of the current of the first secondary winding N2, iN2, is curbed when the parasitic capacitance of switches Cr is discharging via adding an extra inductor or a saturable reactor between the first secondary winding N2 and the first diode D1 in the prior art of this field. The addition of extra inductor or reactance is actually helpful to achieve the ZVS of main switch S1 and effectively solve the over-heating problem of the switch. However, the extra inductor or reactance will induce quite a lot of iron loss when operating at high frequency. Hence, there is still a space of improvement for solving this problem.
Different from the active-clamp forward-flyback converter, the active-clamp flyback converter does not have the first secondary winding N2 transmitting the energy forwardly. As a result, the resonant current iLr would not become small dramatically so as to influence the ZVS function of main switch S1 when the parasitic capacitance of switches Cr is discharging. In the active-clamp flyback converter, the peak value and the valley value of the resonant current iLr would be determined by the value of the load RL. The resonant current iLr would have relatively larger peak and valley values under the heavy load condition. Therefore, it would facilitate the ZVS of main switch S1 under the heavy load condition.
For effectively improving the ZVS scheme of active-clamp forward-flyback converter under the heavy load condition, using the ZVS design of the active-clamp flyback converter is a feasible strategy. Besides, due to the requirements of load variation in a wide range, the efficiency performance at light load has been getting more and more attention. Since the two sub-circuits of the secondary side are both conducted, more conduction losses are correspondingly induced under the light load operation. In the specification of the voltage regulator module (VRM) newly published by Intel (VRM/EVRD 11.1), the requirements for improving the efficiency under the light load condition are set out. Although this kind of specifications are not adopted in the communication systems or in other industrial fields using the isolated power converters, this requirement is most likely to be proposed in the near future.
Keeping the drawbacks of the prior arts in mind, and employing experiments and research full-heartily and persistently, the applicant finally conceived a forward-flyback converter with an active-clamp circuit.